Volume 17, Number 4
Sep 1993

The Role of Alum in Historical Papermaking

by Irene Brückle
Visiting Lecturer in Paper Conservation, Art Conservation Department,
State University College at Buffalo

(This article is based on a talk presented at the IPC Conference
1992 in Manchester, England. A paper published in the conference
proceedings contains additional information not included here.)

As a sizing ingredient, alum has been one of the most important
materials in the history of papermaking. Since the late 19th
century, it has also been mentioned as a primary cause of paper
degradation, but little detailed information on the manufacture and
use of alum has been available in the readily accessible literature.
The two major alum varieties employed in papermaking have not always
been distinguished for their different properties. Aluminum
potassium sulfate was used throughout the history of papermaking
until the 19th century. It was then replaced by the newly developed
aluminum sulfate, a cheaper and more concentrated source of aluminum
compounds. Although both aluminum potassium sulfate and aluminum
sulfate tended to introduce different impurities into paper, the
negative effect of aluminum sulfate on paper degradation overall is
more significant. Certainly, an understanding of the
characteristics of the two compounds can provide certain insights
into the aging properties of paper containing either alum variety.

A. ALUMINUM POTASSIUM SULFATE

Aluminum potassium sulfate had been imported into Europe since
antiquity for application in various trades such as fabric dyeing,
and it was the first alum used in papermaking. It could be obtained
from minerals such as alunite which occurred in sulfur-containing
volcanic sediments. Mining sites were sometimes located in volcanic
crater bottoms where the stones were extracted with naturally heated
water, alum crystals forming in the evaporating solution.

A.1 Alum Production

In the mid-15th century, the first European alum mines were
exploited at Tolfa, a volcanic area north of Rome in central Italy.
This particular site is of interest to us as the source of what
appears to have been one of the best varieties of alum in
papermaking: the so-called Roman alum.1 Known for its high quality until the 19th
century, Roman alum was recognizable by its distinct reddish
appearance, which was a result of dusting the alum crystals with a
pigment--probably iron oxide.2 The
pigmentation served as a trademark of Roman alum. Before use, the
alum could be rinsed under cold water to remove the pigment without
dissolving significant amounts of the alum itself.

Slate and shale were other minerals which yielded alum when
subjected to several production steps which can be summarized as
follows: the aluminous rocks were piled up, roasted, and
subsequently extracted in water; potassium hydroxide was added to
the resulting solution; the crude alum crystals which formed in the
evaporating solution were rinsed and redissolved in boiling water to
purify the alum; the solution was transferred to large wooden casks
where the alum crystals formed on the inside walls; and finally the
casks were dismantled and the crystals removed.3

Alum could be contaminated with byproducts of its manufacture,
iron oxides and iron sulfates. Iron compounds significantly
impaired the performance of alum as a mordant of textile dyes and
were more likely to discolor gelatin-alum sizes, as is indicated by
the concern of 18th-century papermakers for good quality alums. The
repeated recrystallization of the alum effectively freed it from
iron contaminants.

A.2 Historical Gelatin-Alum Sizing

In Europe, the use of alum for hardening gelatin sizes is
recorded during the 16th century. In 1579, Samuel Zimmermann in
Germany wrote in a treatise on so-called secret arts, which included
papermaking: "The printing and other paper. . . is drawn through
alum-water and dried again."4 This
quotation apparently refers to the separate application of gelatin
and alum, a practice which was continued until the 18th
century.5 In the process, the paper
was first gelatin- sized, then steeped in a vessel containing the
alum solution, and finally pressed for removal of excess size.

Gelatin solutions already containing the alum were more
economical to use. They offered the advantage of preventing the
rapid spoilage of the size during storage since the alum crystals
were added to the freshly cooked gelatin sizing solution. Despite
these advantages of the latter method, both sizing processes appear
to have been used contemporaneously.

Like other 18th century writers, Lalande provides more detailed
information on alum than is available from earlier sources. Roman
alum, he states, was preferred in papermaking over what was
designated only as "rock alum."6 Rock
alum was a general term for an inferior quality alum which could
affect the paper brightness. Until the 19th century, paper mills
prepared their own gelatin size solutions by cooking raw animal
parts. The reported weight of alum added to such a solution was
typically based on the weight of the animal parts rather than the
gelatin concentration of the solution. This information only allows
an estimate of the ratio of alum and gelatin in the size.
Percentages of 5% to 40% have been mentioned in the literature.
Barrett used Lalande's text to calculate that the size described in
his book could have had an alum content of 27%, based on the weight
of the dry gelatin.7 It is interesting
to note that aluminum ammonium sulfate was a not uncommon substitute
for regular alum in paper sizing until the 1800s.8

A.3 Miscellaneous Historical Uses of Alum

Watercolorists brushed their gelatin-alum sized papers with an
alum solution to ensure or improve their uniform moisture
resistance. Alum was also believed to give additional luster to
watercolors, as is noted in a 17th century manual.9 More significant, however, is the fact that
some pigments used in watercolors are altered through the increased
acidity of the alum-treated paper surface. In 1901, Church described
how an artist can test the acidity of watercolor papers by applying
ultramarine, chrome yellow and carmine washes. The subsequent
visible alteration of pigment color indicated the presence of excess
alum.10

In traditional bookbinding, alum was also widely used. For
example, decorative paste papers, which were manufactured from the
16th century onward, were sometimes brushed with an alum solution
after application of the pigment-paste mixture. This treatment
increased the toughness of the paper surface.

For centuries, alum has been used as an ingredient in Japanese
dosa or gelatin sizing for the preparation of paper for painting or
printing.

B. ALUMINUM SULFATE

Aluminum sulfate, also called alum, became an industrial product
in the 19th century. It was made by treating either bauxite or
china clay with sulfuric acid. Unlike true alum, aluminum sulfate
could not be conveniently purified through recrystallization because
of its greater solubility in water. This is one of the reasons why
it often contained varying proportions of silica, iron and free
sulfuric acid. By the early 20th century, however, commercial
aluminum sulfate varieties were relatively uniform in quality. They
were ranked according to grading systems and could be purchased in
solid pieces as so-called "patent alum." Well-known varieties
included cake alum11, porous
alum12 and Turkey Red Alum.13 Because of its greater concentration of
alumina (Al2O3) and cheaper production
procedures, aluminum sulfate saved mill expenses and therefore
replaced aluminum potassium sulfate for most purposes in papermaking
and particularly in rosin sizing towards the mid-19th century. This
earned it the name "papermaker's alum." Some 19th century European
papermakers produced alum in their own facilities to ensure a more
consistent quality at a lower price.14 At the mill, alum was emptied into tanks
for dissolution in water prior to use.

B.1 Rosin-Alum Sizing

In the early 19th century, papermakers learned the correct use of
the newly-developed rosin sizing technology by trial and error
methods. The first valid theory explaining the principles of rosin
sizing was not put forward until 1879. To precipitate one part
rosin onto the paper fibers, 1.5 parts alum were required in the
pulp solution, but an excess of alum was frequently added to ensure
sufficient sizing. (Through improvements in 20th-century paper
manufacture, the proportion of rosin and alum added to the pulp has
significantly declined.) Not all of the alum was retained by the
paper and some mills recovered it from the white water drained from
the wire of the paper machine. Papers which needed to be
particularly strong could be sized twice--either with rosin and a
subsequent gelatin size or with two gelatin size applications. In
such cases, the alum concentration was sometimes increased in the
second size bath.

In 19th-century paper mill operations, alum was introduced for
miscellaneous purposes besides sizing. It became what a critic
called "one of the most used (and most abused!) non-fibrous
materials" in the papermaking industry.15

For water clarification, blocks of alum were placed at the bottom
of water tanks feeding into the mill. There the slowly dissolving
alum precipitated suspended dirt particles and alkaline earth
carbonates. The presence of either was particularly undesirable in
rosin sizing.16

In the chlorine bleaching of pulp, small quantities of alum were
added to speed up the chemical reactions. The risk of damaging the
cellulose fibers increased through this treatment.17

Alum helped to retain clay fillers, pigments, resins and fiber
fines in the pulp by aggregation. They were otherwise easily
drained away with the white water from the pulp on the paper
machine.18

Alum was effective in pitch control. Pitch, composed of resinous
unsaponifiable chemicals in certain types of pulp, accumulates in
the headbox and on the wire of the paper machine and can interrupt
its smooth operation. The frothing or foaming of dispersed sizing
materials on the paper machine was also inhibited through the
presence of alum.19

Aluminum ions also act as deflocculating agents in pulp
slurries.20 They react with the paper
fibers and give them electrical charges of the same sign. The
repulsion of their like charges keeps the fibers apart and in even
suspension. On the other hand, alum helped to increase bonding
between fibers during sheet formation on the paper machine. This
increased the wet strength of the paper.

Figure 1. Presence of alum in historic book papers.
The percent of books that contained alum is indicated for each
century. Average pH of tested books is indicated in each bar.
(Source: Barrow.Research Laboratory, 1974, Table 2)

Overall, the use of alum increased between the 16th and the 20th
century. (Fig. 1) Simultaneously, the average pH of book papers
shifted from 6.7 to 4.8.21 Barrett,
in his X-ray fluorescence analysis of book papers from 1500 to 1800,
found that papers in good condition contained less aluminum,
potassium and sulfur than those in poor condition.22 Twentieth-century sizing trends can be
further exemplified by the postwar US rosin consumption, which
steeply increased during the late 1950s.23 At the same time, up to 500,000 tons of
alum were used each year worldwide by the paper industry, which
amounted to 60% of the worldwide alum production. Only a small
percentage of this alum was sold as being iron-free.24 The use of rosin decreased significantly
during the 1980s.

C. ASPECTS OF THE ROLE OF ALUM IN PAPER SIZING

Aluminum ions show a characteristic strong affinity for cellulose
and a complex behavior under various papermaking conditions. Some of
the relevant research of the past years has been excerpted as
follows:

The basic mechanism of rosin sizing involves, among a variety of
other possible reactions, the formation of cationic aluminum salts
from the rosin soap and the aluminum ions. As Gess observes, the
aluminum salt "can then react with the cellulose to provide the
bridged complex" between the cellulose and the rosin.25 (Fig. 2)

In the sizing process with alkyl ketene dimers, the aluminum in
the beta-keto acid complex serves a similar purpose by linking the
dimer to the cellulose, as recently proposed by Gess. (Fig. 3) Only
five pounds of alum are usually added per ton of paper
produced.26

In gelatin-alum sizing, the formation of crosslinkages between
hydrated forms of aluminum ions and gelatin molecules has been
suggested.27

Figure 4. Retention of rosin and alum in paper as a
function of pH conditions during sheet formation. (This graph shows
two of the curves in Fig. 1.1, p. 28, from E. Strazdins, "Chemistry
and Application of Rosin Size," in W.F. Reynolds, ed., The
Sizing of Paper, 2nd ed., Tappi Press, 1989.)

The alum and rosin retention in the paper is determined, among
other aspects, by the pH of the pulp solution. There is evidence
that the alum retention increases with increasing pH in the 4 to 6
range, whereas the rosin retention remains constant over that
range.28 (Fig. 4) Varying pH
conditions also determine the extractability of acidity from paper
as demonstrated for softwood pulp impregnated with aluminum sulfate.
The amount of acidity, measured as sulfur trioxide, which could be
extracted from the sheets was greatest when the sheets were formed
at pH levels between 4 and 6.29
(Fig. 5)

D. CONSERVATION RESEARCH

The effects of conservation treatments on alum-containing papers
have been investigated by a number of individuals. Some of the
findings can be briefly summarized as follows:

In the 1970s Daniels found that the bleach chloramine-T was
retained by water-rinsed test samples impregnated with alum, whereas
alum did not affect the removal of other oxidizing bleaches.30

Wilson noted that, among selected deacidification agents, only
magnesium carbonate was able to reduce the concentration of aluminum
compounds in alum-impregnated papers, and its effect was
slight.31

Prolonged aqueous conservation treatments may cause the partial
removal of alum-containing sizes, as has been noted in a recent
study on the light bleaching of paper carried out at the
Conservation Analytical Laboratory, Smithsonian Institution,
Washington.32

E. EXPERIMENTAL OBSERVATIONS ON GELATIN-ALUM SIZING

In order to investigate the natural aging characteristics of
gelatin-alum sized papers, the author carried out several
experiments. Test papers were sized with 3% gelatin solutions
containing varying proportions of alum. In some cases alkaline
earth carbonates were added to simulate hard water papermaking
systems. The papers were artificially aged at 90°C and 50% RH
and tested for a change of physical properties. The resulting color
change, ÆE, and tensile energy absorption, TEA, of the aged
papers can be summarized as follows:

The increase in discoloration in units of ÆE (measured with
a Minolta Chroma Meter) of sized and aged cotton linter papers was
directly proportional to the increase in aluminum sulfate
concentration, which ranged from 0 to 90% based on the gelatin
content. The alum concentrations above 50% do not reflect
papermaking practices but were included to show possible paper aging
trends resulting from the alum increase. (Fig. 6)

The presence of iron compounds with the alum in the paper
appeared to play a significant simultaneous role in inducing the
color changes. Also, alum concentrations higher than 40% made the
paper very water absorbent.

Accelerated aging (90°C, 50% RH) produced a color shift in
cotton linter papers that depended on the composition of their size.
Under the aging conditions employed, unsized paper remained
relatively white. Gelatin-sized paper showed a tendency towards
yellowish discoloration, whereas increasing alum concentrations in
the gelatin size shifted the discoloration from yellowish towards
grayish brown. Test papers impregnated only with alum also
discolored to a distinct grayish brown tone.

The value of the TEA measure of paper strength declined
proportionally with an increase in alum content of gelatin-alum
sized and aged Whatman filter papers. (Tensile energy absorption is
a measure for the amount of energy necessary to stretch a sheet of
paper to the point of rupture.) Interestingly, the test paper sized
with only 1% alum on the weight of the gelatin did not suffer
physical strength loss during aging. Papers impregnated with
magnesium carbonate or calcium carbonate and then sized with a
gelatin-alum solution showed better physical strength properties
after aging than did similarly sized unimpregnated papers. (Fig. 7)

F. SUMMARY AND CONCLUSIONS

Pre-16th century accounts of the use of alum in papermaking are
scarce, but alum was very likely known to papermakers by the 14th
century. The presence of elemental aluminum and potassium detected
in 15th-century papers indicates the application of alum. The early
use of the substance is not surprising since it provided the most
effective method of reducing the ink absorbency of gelatin sized
writing papers. Papers sized only with gelatin remain readily
moisture absorbent.

In sizing practice, alum was added to the gelatin size or,
alternatively, was applied to paper separately after gelatin sizing.
Both of these sizing methods are recorded in the 18th-century
literature. Whether the aging properties of sized papers were
affected by the method chosen remains to be investigated. The
quality of the alum (as mainly determined by the extent of its
contamination by iron) and of the gelatin probably was more
important in influencing the aging properties of papers than was the
method of application.

Aluminum ammonium sulfate was an occasional substitute for
aluminum potassium sulfate in paper sizing. This should be borne in
mind when elemental analysis of paper specimens shows the presence
of aluminum, but not of potassium.

In sized and artificially aged test papers, alum decreased the
paper strength and brightness. It was observed that, whereas the
use of Whatman filter papers ensured the greater homogeneity of the
sample population and therefore greater reliability of the test
results, handmade cotton linter papers had aging properties more
closely resembling those of naturally aged papers. The cotton
linter papers, for example, discolored more readily upon artificial
aging (90°C, 50% RH), probably because of impurities such as
iron compounds in the pulp.

In the evaluation of historical papers, however, alum should not
be regarded as the only cause of paper degradation. The effect of
alum on paper can only be appreciated in context with the many
interrelated aspects of paper composition, including the quality of
the gelatin used; the pH of the sizing solution; the purity of the
alum; and the presence of other pulp constituents. Although it will
not be possible for a conservator to distinguish between these
aspects for every paper examined, the history of alum application
should be a reminder of the variability of paper as a historical
medium.

Acknowledgements

The author expresses her gratitude to the faculty of the Art
Conservation Department, State University College at Buffalo,
especially to Cathleen Baker for suggestions and comments and to Dan
Kushel and Christopher Tahk for technical advice and assistance
throughout the project. She would also like to thank Timothy
Barrett, Director of Paper Facilities (at the School of Art and Art
History, University of Iowa and the UI Center for the Book), for
carrying out X-ray fluorescence analysis and folding endurance
testing. He, along with Pamela Spitzmueller, University
Conservator, University of Iowa Libraries, generously shared their
insights into papermaking practices and paper properties. The
helpful historical literature suggestions provided by Tom Conroy in
Berkeley, California, are also much appreciated. This paper was
prepared during 1990-91 while the author was a Getty Senior Fellow
at the Art Conservation Department, State University College at
Buffalo.

The Manufacture of Pulp and Paper, Vol. III. (Joint
Executive Committee on Vocational Education Representing the Pulp
and Paper Industry of the United States and Canada) McGraw-Hill
Book Co., New York, 1922.